Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure provide methods and apparatuses for video encoding/decoding. In some examples, an apparatus for video decoding includes receiving circuitry and processing circuitry. For example, the processing circuitry receives a bitstream including bits corresponding to residues associated with a prediction of a block in a coding tree unit (CTU) and determines a maximum transform size based on the CTU size. The maximum transform size is used in a partition to form, from the residues, one or more transform blocks that are respectively encoded into the bits. Then, the processing circuitry reconstructs based on the maximum transform size and from the bits the residues associated with the prediction of the block, and then the processing circuitry reconstructs samples of the block based on the reconstructed residues and the prediction of the block.

INCORPORATION BY REFERENCE

This application is a continuation of application Ser. No. 16/827,248,filed Mar. 23, 2020, which claims the benefit of priority to U.S.Provisional Application No. 62/824,019, “COUPLED CTU SIZE AND MAXIMUM TUSIZE” filed on Mar. 26, 2019. The disclosures of the prior applicationsare hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes receiving circuitry and processing circuitry. For example, theprocessing circuitry receives a bitstream including bits correspondingto residues associated with a prediction of a block in a coding treeunit (CTU) and determines a maximum transform size based on the CTUsize. The maximum transform size is used in a partition to form, fromthe residues, one or more transform blocks that are respectively encodedinto the bits. Then, the processing circuitry reconstructs based on themaximum transform size and from the bits the residues associated withthe prediction of the block, and then the processing circuitryreconstructs samples of the block based on the reconstructed residuesand the prediction of the block.

In some embodiments, the processing circuitry scales the CTU size by ascaling factor to determine the maximum transform size. In an example,the scaling factor is predetermined. In another example, the scalingfactor is determined based on information in the bitstream.

In an example, the processing circuitry divides the CTU size by anallowable number for transform units in the CTU to determine the maximumtransform size.

In some embodiments, the processing circuitry determines the maximumtransform size based on the block size. In an example, the processingcircuitry determines, when the prediction of the block is an intraprediction, the maximum transform size as a minimum of the block sizeand an allowable maximum transform size for intra coded blocks. Inanother example, the processing circuitry determines, when theprediction of the block is an inter prediction, the maximum transformsize as a minimum of the block size and an allowable maximum transformsize for inter coded blocks. In some embodiments, the processingcircuitry determines the maximum transform size based on a smaller sizeof a weight and a height of the block.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the method forvideo decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a simplified block diagram of acommunication system (100) in accordance with an embodiment.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 5 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 6 shows a block diagram of a decoder in accordance with anotherembodiment.

FIGS. 7A-7B show an example of CTU partition.

FIGS. 8A-8B show examples of triple tree partition.

FIGS. 9-12 show transform core matrices in some examples.

FIGS. 13A-13E show a 64×64 transform core matrix.

FIG. 14 shows transform basis functions of the selected DST/DCTtransforms.

FIG. 15 shows a table illustrating a mapping relationship betweenindices and transform types.

FIGS. 16-19 show transform core matrices of DST-7 transform.

FIGS. 20-23 show transform core matrices of DCT-8 transform.

FIG. 24 shows the number of sub-partitions depending on the block size.

FIG. 25 shows the scenario where a block is partitioned into twosub-partitions.

FIG. 26 shows the scenario where a block is partitioned into foursub-partitions.

FIGS. 27A-27B show an example syntax table that includes related syntaxelements signaled for an ISP coding mode.

FIGS. 28A-28D show examples of sub-block transform (SBT).

FIGS. 29A-29I show specification text of a video coding standard whenSBT is used.

FIG. 30 shows different YUV formats used in some embodiments.

FIG. 31 is a flow chat outlining a process example according to someembodiments of the disclosure.

FIG. 32 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a simplified block diagram of a communication system(100) according to an embodiment of the present disclosure. Thecommunication system (100) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (150). Forexample, the communication system (100) includes a first pair ofterminal devices (110) and (120) interconnected via the network (150).In the FIG. 1 example, the first pair of terminal devices (110) and(120) performs unidirectional transmission of data. For example, theterminal device (110) may code video data (e.g., a stream of videopictures that are captured by the terminal device (110)) fortransmission to the other terminal device (120) via the network (150).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (120) may receive the codedvideo data from the network (150), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (100) includes a secondpair of terminal devices (130) and (140) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (130) and (140)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (130) and (140) via the network (150). Eachterminal device of the terminal devices (130) and (140) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (130) and (140), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 1 example, the terminal devices (110), (120), (130) and(140) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (150) represents any number ofnetworks that convey coded video data among the terminal devices (110),(120), (130) and (140), including for example wireline (wired) and/orwireless communication networks. The communication network (150) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(150) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 2 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (213), that caninclude a video source (201), for example a digital camera, creating forexample a stream of video pictures (202) that are uncompressed. In anexample, the stream of video pictures (202) includes samples that aretaken by the digital camera. The stream of video pictures (202),depicted as a bold line to emphasize a high data volume when compared toencoded video data (204) (or coded video bitstreams), can be processedby an electronic device (220) that includes a video encoder (203)coupled to the video source (201). The video encoder (203) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (204) (or encoded video bitstream (204)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (202), can be stored on a streamingserver (205) for future use. One or more streaming client subsystems,such as client subsystems (206) and (208) in FIG. 2 can access thestreaming server (205) to retrieve copies (207) and (209) of the encodedvideo data (204). A client subsystem (206) can include a video decoder(210), for example, in an electronic device (230). The video decoder(210) decodes the incoming copy (207) of the encoded video data andcreates an outgoing stream of video pictures (211) that can be renderedon a display (212) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (204),(207), and (209) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (220) and (230) can includeother components (not shown). For example, the electronic device (220)can include a video decoder (not shown) and the electronic device (230)can include a video encoder (not shown) as well.

FIG. 3 shows a block diagram of a video decoder (310) according to anembodiment of the present disclosure. The video decoder (310) can beincluded in an electronic device (330). The electronic device (330) caninclude a receiver (331) (e.g., receiving circuitry). The video decoder(310) can be used in the place of the video decoder (210) in the FIG. 2example.

The receiver (331) may receive one or more coded video sequences to bedecoded by the video decoder (310); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (301), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (331) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (331) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (315) may be coupled inbetween the receiver (331) and an entropy decoder/parser (320) (“parser(320)” henceforth). In certain applications, the buffer memory (315) ispart of the video decoder (310). In others, it can be outside of thevideo decoder (310) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (310), forexample to combat network jitter, and in addition another buffer memory(315) inside the video decoder (310), for example to handle playouttiming. When the receiver (331) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (315) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (315) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (310).

The video decoder (310) may include the parser (320) to reconstructsymbols (321) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (310),and potentially information to control a rendering device such as arender device (312) (e.g., a display screen) that is not an integralpart of the electronic device (330) but can be coupled to the electronicdevice (330), as was shown in FIG. 3 . The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (320) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (320) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (320) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (320) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (315), so as tocreate symbols (321).

Reconstruction of the symbols (321) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (320). The flow of such subgroup control information between theparser (320) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (310)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (351). Thescaler/inverse transform unit (351) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (321) from the parser (320). The scaler/inversetransform unit (351) can output blocks comprising sample values, thatcan be input into aggregator (355).

In some cases, the output samples of the scaler/inverse transform (351)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (352). In some cases, the intra pictureprediction unit (352) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (358). The currentpicture buffer (358) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(355), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (352) has generated to the outputsample information as provided by the scaler/inverse transform unit(351).

In other cases, the output samples of the scaler/inverse transform unit(351) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (353) canaccess reference picture memory (357) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (321) pertaining to the block, these samples can beadded by the aggregator (355) to the output of the scaler/inversetransform unit (351) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (357) from where themotion compensation prediction unit (353) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (353) in the form of symbols (321) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (357) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (355) can be subject to variousloop filtering techniques in the loop filter unit (356). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (356) as symbols (321) from the parser (320), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (356) can be a sample stream that canbe output to the render device (312) as well as stored in the referencepicture memory (357) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (320)), the current picture buffer (358) can becomea part of the reference picture memory (357), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (310) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (331) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (310) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 4 shows a block diagram of a video encoder (403) according to anembodiment of the present disclosure. The video encoder (403) isincluded in an electronic device (420). The electronic device (420)includes a transmitter (440) (e.g., transmitting circuitry). The videoencoder (403) can be used in the place of the video encoder (203) in theFIG. 2 example.

The video encoder (403) may receive video samples from a video source(401) (that is not part of the electronic device (420) in the FIG. 4example) that may capture video image(s) to be coded by the videoencoder (403). In another example, the video source (401) is a part ofthe electronic device (420).

The video source (401) may provide the source video sequence to be codedby the video encoder (403) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (401) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (401) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (403) may code andcompress the pictures of the source video sequence into a coded videosequence (443) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (450). In some embodiments, the controller(450) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (450) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (450) can be configured to have other suitablefunctions that pertain to the video encoder (403) optimized for acertain system design.

In some embodiments, the video encoder (403) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (430) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (433)embedded in the video encoder (403). The decoder (433) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (434). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (434) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (433) can be the same as of a“remote” decoder, such as the video decoder (310), which has alreadybeen described in detail above in conjunction with FIG. 3 . Brieflyreferring also to FIG. 3 , however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (445) and the parser (320) can be lossless, the entropy decodingparts of the video decoder (310), including the buffer memory (315), andparser (320) may not be fully implemented in the local decoder (433).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (430) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (432) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (433) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (430). Operations of the coding engine (432) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 4 ), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (433) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (434). In this manner, the video encoder(403) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (435) may perform prediction searches for the codingengine (432). That is, for a new picture to be coded, the predictor(435) may search the reference picture memory (434) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(435) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (435), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (434).

The controller (450) may manage coding operations of the source coder(430), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (445). The entropy coder (445)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (440) may buffer the coded video sequence(s) as createdby the entropy coder (445) to prepare for transmission via acommunication channel (460), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(440) may merge coded video data from the video coder (403) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (450) may manage operation of the video encoder (403).During coding, the controller (450) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (403) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (403) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (440) may transmit additional datawith the encoded video. The source coder (430) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTUs) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 5 shows a diagram of a video encoder (503) according to anotherembodiment of the disclosure. The video encoder (503) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (503) is used in theplace of the video encoder (203) in the FIG. 2 example.

In an HEVC example, the video encoder (503) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (503) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (503) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(503) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (503) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 5 example, the video encoder (503) includes the interencoder (530), an intra encoder (522), a residue calculator (523), aswitch (526), a residue encoder (524), a general controller (521), andan entropy encoder (525) coupled together as shown in FIG. 5 .

The inter encoder (530) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (522) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (522) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (521) is configured to determine general controldata and control other components of the video encoder (503) based onthe general control data. In an example, the general controller (521)determines the mode of the block, and provides a control signal to theswitch (526) based on the mode. For example, when the mode is the intramode, the general controller (521) controls the switch (526) to selectthe intra mode result for use by the residue calculator (523), andcontrols the entropy encoder (525) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(521) controls the switch (526) to select the inter prediction resultfor use by the residue calculator (523), and controls the entropyencoder (525) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (523) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (522) or the inter encoder (530). Theresidue encoder (524) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (524) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (503) also includes a residuedecoder (528). The residue decoder (528) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (522) and theinter encoder (530). For example, the inter encoder (530) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (522) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (525) is configured to format the bitstream toinclude the encoded block. The entropy encoder (525) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (525) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 6 shows a diagram of a video decoder (610) according to anotherembodiment of the disclosure. The video decoder (610) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (610) is used in the place of the videodecoder (210) in the FIG. 2 example.

In the FIG. 6 example, the video decoder (610) includes an entropydecoder (671), an inter decoder (680), a residue decoder (673), areconstruction module (674), and an intra decoder (672) coupled togetheras shown in FIG. 6 .

The entropy decoder (671) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (672) or the inter decoder (680), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (680); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (672). The residual information can be subject to inversequantization and is provided to the residue decoder (673).

The inter decoder (680) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (672) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (673) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (673) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (671) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (674) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (673) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (203), (403), and (503), and thevideo decoders (210), (310), and (610) can be implemented using anysuitable technique. In an embodiment, the video encoders (203), (403),and (503), and the video decoders (210), (310), and (610) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (203), (403), and (403), and the videodecoders (210), (310), and (610) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure provide techniques for coupling CTU size withmaximum TU size. Specifically, in some embodiments, the disclosureprovides schemes for controlling maximum transform unit size, and forcontrolling the interaction between maximum transform unit size andtransform partitioning schemes (e.g., sub-block transform (SBT) andintra sub partitioning (ISP)).

A block partitioning structure is referred to as a coding tree. In someembodiments, by using a quadtree structure, a coding tree unit (CTU) issplit into coding units (CUs) to adapt to various local characteristics.A decision on whether to code a picture area using an inter-picture(temporal) or intra-picture (spatial) prediction is made at CU level.Each CU can be further split into one, two, or four prediction units(PUs) according to a PU splitting type. Inside one PU, a same predictionprocess is applied and relevant information is transmitted to a decoderon a PU basis.

After obtaining a residual block by applying a prediction process basedon the PU splitting type, a CU can be partitioned into transform units(TUs) according to another quadtree structure. As can be seen, there aremultiple partition conceptions including CU, PU, and TU. In someembodiments, a CU or a TU can only be square shape, while a PU may besquare or rectangular shape. In some embodiments, one coding block maybe further split into four square sub-blocks, and transform is performedon each sub-block, i.e., TU. Each TU can be further split recursivelyinto smaller Tus using a quadtree structure which is called residualquadtree (RQT).

At a picture boundary, in some embodiments, implicit quadtree split canbe employed so that a block will keep quad-tree splitting until the sizefits the picture boundary.

In some embodiments, a quadtree plus binary tree (QTBT) structure isemployed. The QTBT structure removes the concepts of multiple partitiontypes (the CU, PU and TU concepts), and supports more flexibility for CUpartition shapes. In the QTBT block structure, a CU can have either asquare or rectangular shape.

FIG. 7A shows a CTU (710) that is partitioned by using a QTBT structure(720) shown in FIG. 7B. The CTU (710) is first partitioned by a quadtreestructure. The quadtree leaf nodes are further partitioned by a binarytree structure or a quadtree structure. There can be two splittingtypes, symmetric horizontal splitting and symmetric vertical splitting,in the binary tree splitting. The binary tree leaf nodes are called CUsthat can be used for prediction and transform processing without anyfurther partitioning. Accordingly, CU, PU and TU have the same blocksize in the QTBT coding block structure.

In some embodiments, a CU can include coding blocks (CBs) of differentcolor components. For example, one CU contains one luma CB and twochroma CBs in the case of P and B slices of the 4:2:0 chroma format. ACU can include a CB of a single color component. For example, one CUcontains only one luma CB or just two chroma CBs in the case of Islices.

Some parameters are defined for the QTBT partitioning scheme in someembodiments. The parameters include CTU size, MinQTSize, MaxBTSize,MaxBTDepth and MinBTSize. CTUsize defines the root node size of aquadtree, e.g. the same concept as in HEVC. MinQTSize is the minimumallowed quadtree leaf node size. MaxBTSize is the maximum allowed binarytree root node size. MaxBTDepth is the maximum allowed binary treedepth. MinBTSize is the minimum allowed binary tree leaf node size.

In an example of the QTBT partitioning structure, the CTUsize is set as128×128 luma samples with two corresponding 64×64 blocks of chromasamples, the MinQTSize is set as 16×16, theMaxBTSize is set as 64×64,the MinBTSize (for both width and height) is set as 4×4, and theMaxBTDepth is set as 4. The quadtree partitioning is applied to the CTUfirst to generate quadtree leaf nodes. The quadtree leaf nodes may havea size from 16×16 (i.e., theMinQTSize) to 128×128 (i.e., the CTU size).If the leaf quadtree node is 128×128, it will not be further split bythe binary tree since the size exceeds theMaxBTSize (i.e., 64×64).Otherwise, the leaf quadtree node could be further partitioned by thebinary tree. Therefore, the quadtree leaf node is also the root node forthe binary tree and it has the binary tree depth as 0.

When the binary tree depth reaches MaxBTDepth (i.e., 4), no furthersplitting is considered. When the binary tree node has width equaltoMinBTSize (i.e., 4), no further horizontal splitting is considered.Similarly, when the binary tree node has height equal to MinBTSize, nofurther vertical splitting is considered. The leaf nodes of the binarytree are further processed by prediction and transform processingwithout any further partitioning. In an embodiment, a maximum CTU sizeis 256×256 luma samples.

In FIGS. 7A and 7B, the solid lines indicate quadtree splitting anddotted lines indicate binary tree splitting. In each splitting (i.e.,non-leaf) node of the binary tree, one flag is signaled to indicatewhich splitting type (i.e., horizontal or vertical) is used. Forexample, 0 indicates a horizontal splitting and 1 indicates a verticalsplitting. For the quadtree splitting, there is no need to indicate thesplitting type since quadtree splitting always splits a block bothhorizontally and vertically to produce 4 sub-blocks with an equal size.

In some embodiments, the QTBT scheme supports the flexibility for theluma and chroma to have a separate QTBT structure. For example, for Pand B slices, the luma and chroma blocks in one CTU share the same QTBTstructure. However, for I slices, the luma CTB is partitioned into CUsby a QTBT structure, and the chroma blocks are partitioned into chromaCUs by another QTBT structure. Thus, a CU in an I slice consists of acoding block of the luma component or coding blocks of two chromacomponents, and a CU in a P or B slice consists of coding blocks of allthree color components.

In some embodiments, inter prediction for small blocks is restricted toreduce memory access of motion compensation. For example, bi-predictionis not supported for 4×8 and 8×4 blocks, and inter prediction is notsupported for 4×4 blocks.

In some embodiments, a multi-type-tree (MTT) structure is used forpartitioning a picture. The MTT structure is a more flexible treestructure than the QTBT structure. In MTT, in addition to quad-tree andbinary-tree, horizontal center-side triple-tree and vertical center-sidetriple-tree are employed.

FIG. 8A shows an example of vertical center side triple tree partition.The vertical center side triple tree partition split a block verticallyinto three smaller blocks. The center block is twice the size of each ofthe side blocks.

FIG. 8B shows an example of horizontal center-side triple-treepartition. The horizontal center-side triple tree partition split ablock horizontally into three smaller blocks. The center block is twicethe size of each of the side blocks.

Triple tree partitioning can complement quad-tree and binary-treepartitioning. For example, triple-tree partitioning is able to captureobjects which locate in a block center, while quad-tree and binary-treesplits crossing block centers. The width and height of partitions bytriple trees are a power of 2 so that no additional transform partitionis needed.

According to some aspects of the disclosure, primary and secondarytransforms can be used in the coding of residual signals. In someembodiments, 4-point, 8-point, 16-point and 32-point type 2 discretecosine transform (DCT-2) transforms can be used as primary transform.FIGS. 9-12 show transform core matrices of 4-point, 8-point, 16-point,and 32-point DCT-2, respectively. Elements of those transform corematrices can be represented using 8-bit integers, and thus thosetransform core matrices are referred to as 8-bit transform cores. Asshown, the transform core matrix of a smaller DCT-2 is a part of that ofa larger DCT-2.

The DCT-2 core matrices show symmetry/anti-symmetry characteristics.Accordingly, a so-called “partial butterfly” implementation can besupported to reduce the number of operation counts (multiplications,adds/subs, shifts). Identical results of matrix multiplication can beobtained using the partial butterfly implementation.

In some embodiments, in addition to 4-point, 8-point, 16-point and32-point DCT-2 transforms described above, additional 2-point and64-point DCT-2 are used. FIGS. 13A-13E show a 64×64 transform corematrix of the 64-point DCT-2 transform.

In some embodiments, in addition to DCT-2 and 4×4 discrete sinetransform type IIV (DST-7) transforms, an adaptive multiple transform(AMT) (also known as enhanced multiple transform (EMT), or multipletransform selection (MTS)) is used for residual coding of both inter andintra coded blocks. The AMT uses multiple selected transforms fromdiscrete cosine transform (DCT)/discrete sine transform (DST) familiesin addition to DCT-2 transforms, such as transform core matrices ofDST-7, or DCT-8 transform.

FIG. 14 shows transform basis functions of the selected DST/DCTtransforms.

In some embodiments, the DST/DCT transform core matrices used in AMT arerepresented with 8-bit representation. In some embodiments, AMT isapplied to CUs with both width and height smaller than or equal to 32.Whether to apply AMT or not can be controlled by a flag denoted bymts_flag. For example, when the mts_flag is equal to 0, only DCT-2 isapplied to coding a residue block. When the mts_flag is equal to 1, anindex, denoted by mts_idx, can further be signaled using 2 bins tospecify a horizontal and vertical transforms to be used.

FIG. 15 shows a table (1500) illustrating a mapping relationship betweenan mts_idx value and respective horizontal or vertical transforms. Therow (1501) with the mts_idx having a value of −1 corresponds to thescenario where the mts_flag is equal to 0, and DCT-2 transform is used.The rows (1502)-(1505) with the mts_idx having a value of 0, 1, 2, or 3correspond to the scenario where the mts_flag is equal to 1. In theright two columns of the table (1500), 0 represents a transform type ofDCT-2, 1 represents a transform type of DST-7, and 2 represents atransform type of DCT 8.

FIGS. 16-19 show transform core matrices of DST-7 transform. FIGS. 20-23show transform core matrices of DCT-8 transform.

In some examples, such as in VVC, when both the height and width of thecoding block is smaller than or equal to 64, the transform size is thesame as the coding block size. When either the height or width of thecoding block is larger than 64, when doing a transform (such as aninverse transform, an inverse primary transform, or the like) or intraprediction, the coding block is further split into multiple sub-blocks,where the width and height of each sub-block is smaller than or equal to64. One transform can be performed on each sub-block.

In some embodiments, an intra sub-partition (ISP) coding mode isemployed. In ISP coding mode, a luma intra-predicted block can bepartitioned vertically or horizontally into 2 or 4 sub-partitions. Thenumber of sub-partitions can depend on a size of the block. FIG. 24shows the number of sub-partitions depending on the block size. FIG. 25shows the scenario where a block is partitioned into two sub-partitions.FIG. 26 shows the scenario where a block is partitioned into foursub-partitions. In an example, all sub-partitions fulfill a condition ofhaving at least 16 samples. In an example, ISP is not applied to chromacomponents.

In an example, for each of sub-partitions partitioned from a codingblock, a residual signal is generated by entropy decoding respectivecoefficients sent from an encoder and then inverse quantizing andinverse transforming them. Then, a first one of the sub-partitions isintra predicted to generate a prediction signal. The prediction signalis added to the respective residual signal of the first sub-partition toobtain corresponding reconstructed samples. Thereafter, thereconstructed sample values of the first sub-partition can be availableto generate a prediction of a second one of the sub-partitions. Thisprocess can be repeated sub-partition by sub-partition, until allsub-partitions from the coding block are reconstructed. In an example,all the sub-partitions share a same intra mode.

In an embodiment, the ISP coding mode is only tested with intra modesthat are part of a most probable mode (MPM) list. Accordingly, if ablock uses ISP, then a MPM flag can be inferred to be one. In addition,when ISP is used for a certain block, then a respective MPM list will bemodified to exclude DC mode and to prioritize horizontal intra modes forthe ISP horizontal split and vertical intra modes for the vertical one.

In ISP coding mode, each sub-partition can be regarded as a sub-TU,since the transform and reconstruction is performed individually foreach sub-partition.

FIGS. 27A-27B show an example syntax table (2700) that includes relatedsyntax elements signaled for an ISP coding mode. As shown in the portion(2710), a syntax element, intra_subpartitions_mode_flag, indicateswhether ISP is used or not. A syntax element,intra_subpartitions_split_flag, indicates a partition direction(vertical or horizontal).

In some embodiments, a sub-block transform (SBT), also referred to asspatially varying transform (SVT), is employed. The SBT can be appliedto inter prediction residuals. For example, in a coding block, only aportion of the coding block is treated as a residual block, andtransform is applied to the portion of the coding block. Zero residualis assumed for the remaining portion of the coding block. Therefore, theresidual block is smaller than the coding block, and a transform size inSBT is smaller than the coding block size. For the region which is notcovered by the residual block, no transform processing is performed.

FIGS. 28A-28D show sub-block types (SVT-H, SVT-V) (e.g., vertically orhorizontally partitioned), sizes and positions (e.g., left half, leftquarter, right half, right quarter, top half, top quarter, bottom half,bottom quarter) supported in SBT. The shaded regions labeled by letter“A” is residual blocks with transform, and the other regions is assumedto be zero residual without transform.

As an example, FIGS. 29A-29I show changes to a specification text of avideo coding standard (e.g., VVC) being developed by the Joint VideoExperts Team (JVET) when SBT is used. The added texts are shown inframes from (2901) to (2913). As shown, additional syntax elements,cu_sbt_flag, cu_sbt_quad_flag, cu_sbt_horizontal_flag, andcu_sbt_pos_flag, are signaled to indicate the sub-block type (horizontalor vertical), size (half or quarter) and position (left or right, top orbottom), respectively.

FIG. 30 shows different YUV formats (e.g., 4:4:4, 4:2:2, 4:1:1, and4:2:0) used in some embodiments. In an example, a cross component linearmodel intra prediction is used for the 4:2:0 format. A six-tapinterpolation filter can be applied to obtain a down-sampled luma samplecorresponding to a chroma sample as shown in FIG. 30 . In a formula way,a down-sampled luma sample Rec′L[x, y] can be calculated from nearbyreconstructed luma samples (represented by Rec_(L)[x, y]) according to(Eq. 1):Rec′ _(L)[x,y]=(2×Rec _(L)[2x,2y]+2×Rec _(L)[2x,2y+1]+Rec_(L)[2x−1,2y]+Rec _(L)[2x+1,2y]+Rec _(L)[2x−1,2y+1]+Rec_(L)[2x+1,2y+1]+4)>>3  (Eq. 1)

According to some aspects of the disclosure, the maximum TU size mayhave interaction with other coding techniques and may conflict withrequirements in the some coding tools. Generally, a CU may bepartitioned into TUs based on maximum TU size.

In some examples, such as in some versions of VVC draft, the maximum TUsize is a fix number 64, thus there is no control on the maximum TUsize. Some encoders or decoders are implemented using pipelinearchitecture. The pipeline intermediate buffer size is closed related tothe maximum TU size. When the maximum TU size is fixed withoutflexibility, the implementation of the encoders and decoders in someexisting pipeline architecture hardware may be complicate. Thus, thereis a requirement of controlling maximum TU size in VVC since maximum TUsize has an impact on hardware complexity for encoder implementation.

In some examples, such as in some versions of VVC draft, certain codingtechniques, such as SBT and ISP, can be used. The implementation of theSBT and ISP coding techniques may have dependency on the maximum TUsize. The interactions between SBT, ISP and maximum TU size need to behandled. In an example, according to SBT, a SPS flagsps_sbt_max_size_64_flag is signaled to indicate whether the largest SBTsize is 32 (samples) in length or 64 (sample) in length. When the SPSflag sps_sbt_max_size_64_flag is true, a sub-block for SBT may have asize of 32×64 for example. When the maximum TU size is 32, the encodercannot handle and thus an encoder crash may be triggered.

In some examples, ISP mode is allowed for all CU sizes. However, whenmax transform size is set to be smaller than 64, there is a conflictwhether implicit transform split or explicit transform split using ISPwith signaling should be performed. For example, the maximum transformsize is 16. For a 64×16 block, without ISP, the block is implicitlysplit into four 16×16 TUs. However, with ISP, the block may bepartitioned using vertical ISP splitting, which results into same four16×16 TUs, but using signaling.

In some examples, multiple TUs within one CU is allowed. However, whenthe CTU size is large, the CTU may be partitioned based on a relativelysmall maximum transform size, and thus the result of the partition canbe a relatively large number of TUs. The large number of TUs may be nothardware implementation friendly. For example, in some implementation,up to four TUs needs to be processed within one CU. However, when themax TU size and CTU size is completely decoupled, then for a 128×128 CUand 16×16 max TU size, the total number of TUs can be 64 TUs.

The proposed methods may be used separately or combined in any order.Further, each of the methods (or embodiments), encoder, and decoder maybe implemented by processing circuitry (e.g., one or more processors orone or more integrated circuits). In one example, the one or moreprocessors execute a program that is stored in a non-transitorycomputer-readable medium.

In the following, the term block may be interpreted as a predictionblock, a coding block, or a coding unit, i.e. CU. In this document, whensaying high-level syntax element, it may refer to any of Video ParameterSet (VPS), Sequence Parameter Set (SPS), Picture Parameter Set (PPS),Slice header, Tile header, Tile group header. When saying CTU (codingtree unit, which is the largest CU size) header, it refers to syntaxelements signaled for each CTU, e.g., as header information. In thisdocument, when saying transform size, it may refer to the maximumtransform width and/height, or maximum transform unit area size.

According to an aspect of the disclosure, the maximum transform size(maxTU, and is also referred to as maximum TU size) is derived based onthe CTU size. In an example, the CTU size is defined as the larger oneof the CTU width and CTU height, such as max(CTU_width, CTU_height).Then, the maximum transform size maxTU is determined by scaling the CTUsize by a pre-defined number.

In some embodiments, the maximum TU size is derived as the CTU sizedivided by 2^(N), where example values of N include 1, 2, 3 or 4. Forexample, N is 1, then when the CTU size is 128, the maximum TU size isderived as 64; when the CTU size is 64, the maximum TU size is derivedas 32; when the CTU size is 32, the maximum TU size is derived as 16. Inan example, N may be predefined. In another example, N is signaled inbitstream, such as in high-level syntax.

In another embodiment, the maximum TU size is derived as the CTU sizedivided by the maximum allowed number of TUs inside a CTU (or CU). Insome examples, for different color formats, the maximum allowed numberof TUs may be different for different components. In an example, themaximum allowed number of TUs is 4 for luma component, and the maximumallowed number of TUs can be 4 or 1 for chroma component in 4:2:0contents. In another example, maximum allowed number of TUs is 4 forluma component, and the maximum allowed number of TUs can be 2 forchroma component in 4:2:2 contents. The maximum allowed number of TUsmay be predefined or signaled in bitstream, such as in high-levelsyntax.

According to another aspect of the disclosure, the maximum TU size in acurrent CU is derived based on the CU size, such as derived as the CUsize scaled by a pre-defined number. In some embodiments, intra codedblock and inter coded block may use different maximum TU sizes. Forexample, two allowable maximum TU sizes can be determined. The twoallowable maximum TU sizes are referred to as maxTUintra (allowablemaximum transform size for intra coded block) and maxTUinter (allowablemaximum transform size for inter coded block) in an example. In someexamples, the maxTUintra is limited or defined based on intra codingtechniques or intra coding tools, such as ISP and the like; and themaxTU inter is limited or defined based on inter coding techniques orintra coding tools, such as SBT and the like.

In an embodiment, the maximum TU size for a current CU depends onwhether the current CU is intra coded or inter coded. In an example, forintra coded CU, the maximum TU size allowed for current CU is derived asminimum between the current CU size and the value of maxTUintra. Inanother example, for inter coded CU, the maximum TU size allowed forcurrent CU is derived as minimum between the current CU size and thevalue of maxTUinter.

In an example, the values of maxTUintra and maxTUinter can be signaledin high-level syntax.

In another example, the values of maxTUintra and maxTUinter can bederived based on the CTU size, such as scaling the CTU size bypre-defined numbers.

In another example, the maximum TU size for a current CU depends on theshape of the current CU, such as whether the current CU is square orrectangular, or the aspect ratio of current CU. In an example, for intracoded CU, the maximum TU size allowed for current CU is derived asminimum between the current CU size (width and/or height) and the valueof maximum allowable TU size for intra coded CU. Thus, when the currentCU is square and intra coded, the maximum TU size for the current CU isthe minimum between the width and the value of maximum allowable TU sizefor intra coded CU. When the current CU is rectangular and intra coded,the maximum TU size for the current CU is the minimum between the valueof the maximum allowable TU size for intra coded CU, and the shortersize of the width and the height.

FIG. 31 shows a flow chart outlining a process (3100) according to anembodiment of the disclosure. The process (3100) can be used in thereconstruction of a block, so to generate a prediction block for theblock under reconstruction. In various embodiments, the process (3100)are executed by processing circuitry, such as the processing circuitryin the terminal devices (110), (120), (130) and (140), the processingcircuitry that performs functions of the video encoder (203), theprocessing circuitry that performs functions of the video decoder (210),the processing circuitry that performs functions of the video decoder(310), the processing circuitry that performs functions of the videoencoder (403), and the like. In some embodiments, the process (3100) isimplemented in software instructions, thus when the processing circuitryexecutes the software instructions, the processing circuitry performsthe process (3100). The process starts at (S3101) and proceeds to(S3110).

At (3110), a bitstream is received. The bitstream includes bitscorresponding to residues associated with a prediction of a block in aCTU. For example, at an encoder side, residues of a prediction block tothe original block are calculated. The block of residues is partitionedinto transform blocks. Transforms are applied to the transform blocks togenerate coefficients. The coefficients are entropy coded into bits,thus the bits correspond to the residues associated with the predictionof the block. The bits are included into the bitstream at the encoderside.

At (3120), a maximum transform size is determined based on the CTU size.The maximum transform size is used in a partition to form, from theresidues, one or more transform units that are respectively coded intothe bits.

At (3130), the residues are reconstructed from the bits based on themaximum transform unit.

At (3140), samples of the block are reconstructed based on thereconstructed residues and the prediction of the block. The processproceeds to (3199) and terminates.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 32 shows a computersystem (3200) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 32 for computer system (3200) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (3200).

Computer system (3200) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (3201), mouse (3202), trackpad (3203), touchscreen (3210), data-glove (not shown), joystick (3205), microphone(3206), scanner (3207), camera (3208).

Computer system (3200) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (3210), data-glove (not shown), or joystick (3205), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (3209), headphones(not depicted)), visual output devices (such as screens (3210) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability-some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (3200) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(3220) with CD/DVD or the like media (3221), thumb-drive (3222),removable hard drive or solid state drive (3223), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (3200) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (3249) (such as, for example USB ports of thecomputer system (3200)); others are commonly integrated into the core ofthe computer system (3200) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (3200) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (3240) of thecomputer system (3200).

The core (3240) can include one or more Central Processing Units (CPU)(3241), Graphics Processing Units (GPU) (3242), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(3243), hardware accelerators for certain tasks (3244), and so forth.These devices, along with Read-only memory (ROM) (3245), Random-accessmemory (3246), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (3247), may be connectedthrough a system bus (3248). In some computer systems, the system bus(3248) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (3248),or through a peripheral bus (3249). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (3241), GPUs (3242), FPGAs (3243), and accelerators (3244) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(3245) or RAM (3246). Transitional data can be also be stored in RAM(3246), whereas permanent data can be stored for example, in theinternal mass storage (3247). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (3241), GPU (3242), massstorage (3247), ROM (3245), RAM (3246), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (3200), and specifically the core (3240) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (3240) that are of non-transitorynature, such as core-internal mass storage (3247) or ROM (3245). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (3240). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(3240) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (3246) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (3244)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

-   JEM: joint exploration model-   VVC: versatile video coding-   BMS: benchmark set-   MV: Motion Vector-   HEVC: High Efficiency Video Coding-   SEI: Supplementary Enhancement Information-   VUI: Video Usability Information-   GOPs: Groups of Pictures-   TUs: Transform Units,-   PUs: Prediction Units-   CTUs: Coding Tree Units-   CTBs: Coding Tree Blocks-   PBs: Prediction Blocks-   HRD: Hypothetical Reference Decoder-   SNR: Signal Noise Ratio-   CPUs: Central Processing Units-   GPUs: Graphics Processing Units-   CRT: Cathode Ray Tube-   LCD: Liquid-Crystal Display-   OLED: Organic Light-Emitting Diode-   CD: Compact Disc-   DVD: Digital Video Disc-   ROM: Read-Only Memory-   RAM: Random Access Memory-   ASIC: Application-Specific Integrated Circuit-   PLD: Programmable Logic Device-   LAN: Local Area Network-   GSM: Global System for Mobile communications-   LTE: Long-Term Evolution-   CANBus: Controller Area Network Bus-   USB: Universal Serial Bus-   PCI: Peripheral Component Interconnect-   FPGA: Field Programmable Gate Areas-   SSD: solid-state drive-   IC: Integrated Circuit-   CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video encoding in a video encoder,comprising: calculating, by processing circuitry of the video encoder,residues associated with a prediction of a block in a coding tree unit(CTU); determining, by the processing circuitry, a maximum transformsize based on a size of the CTU; forming, by the processing circuitryand based on the determined maximum transform size, one or moretransform blocks from the calculated residues; applying, by theprocessing circuitry, at least one transform to each of the formed oneor more transform blocks to generate transform coefficients; and entropycoding, by the processing circuitry, the generated transformcoefficients into bits of a coded video bitstream.
 2. The method ofclaim 1, further comprising: scaling, by the processing circuitry, theCTU size by a scaling factor to determine the maximum transform size. 3.The method of claim 2, wherein the scaling factor is predetermined. 4.The method of claim 2, wherein the scaling factor is indicated in thecoded video bitstream.
 5. The method of claim 1, further comprising:dividing, by the processing circuitry, the CTU size by an allowablenumber for transform units in the CTU to determine the maximum transformsize.
 6. The method of claim 1, further comprising: determining, by theprocessing circuitry, the maximum transform size, based on a size of theblock in the CTU.
 7. The method of claim 6, further comprising:determining, by the processing circuitry, when the prediction of theblock is an intra prediction, the maximum transform size as a smallerone of (i) the size of the block in the CTU and (ii) an allowablemaximum transform size for intra coded blocks.
 8. The method of claim 6,further comprising: determining, by the processing circuitry, when theprediction of the block is an inter prediction, the maximum transformsize as a smaller one of (i) the size of the block in the CTU and (ii)an allowable maximum transform size for inter coded blocks.
 9. Themethod of claim 6, further comprising: determining, by the processingcircuitry, the maximum transform size based on a smaller one of a weightand a height of the block in the CTU.
 10. An apparatus for videoencoding, comprising: processing circuitry configured to calculateresidues associated with a prediction of a block in a coding tree unit(CTU); determine a maximum transform size based on a size of the CTU;form, based on the determined maximum transform size, one or moretransform blocks from the calculated residues; apply at least onetransform to each of the formed one or more transform blocks to generatetransform coefficients; and entropy code the generated transformcoefficients into bits of a coded video bitstream.
 11. The apparatus ofclaim 10, wherein the processing circuitry is configured to: scale theCTU size by a scaling factor to determine the maximum transform size.12. The apparatus of claim 11, wherein the scaling factor ispredetermined.
 13. The apparatus of claim 11, wherein the scaling factoris indicated in the coded video bitstream.
 14. The apparatus of claim10, wherein the processing circuitry is configured to: divide the CTUsize by an allowable number for transform units in the CTU to determinethe maximum transform size.
 15. The apparatus of claim 10, wherein theprocessing circuitry is configured to: determine the maximum transformsize based on a size of the block in the CTU.
 16. The apparatus of claim15, wherein the processing circuitry is configured to: determine, whenthe prediction of the block is an intra prediction, the maximumtransform size as a smaller one of (i) the size of the block in the CTUand (ii) an allowable maximum transform size for intra coded blocks. 17.The apparatus of claim 15, wherein the processing circuitry isconfigured to: determine, when the prediction of the block is an interprediction, the maximum transform size as a smaller one of (i) the sizeof the block in the CTU and (ii) an allowable maximum transform size forinter coded blocks.
 18. The apparatus of claim 15, wherein theprocessing circuitry is configured to: determine the maximum transformsize based on a smaller one of a weight and a height of the block in theCTU.
 19. A non-transitory computer-readable storage medium storinginstructions which, when executed by processor of a video encoder, causethe processor to perform: calculating residues associated with aprediction of a block in a coding tree unit (CTU); determining a maximumtransform size based on a size of the CTU; forming, based on thedetermined maximum transform size, one or more transform blocks from thecalculated residues; applying at least one transform to each of theformed one or more transform blocks to generate transform coefficients;and entropy coding the generated transform coefficients into bits of acoded video bitstream.
 20. The non-transitory computer-readable storagemedium of claim 19, wherein the instructions further cause the processorto perform: scaling the CTU size by a scaling factor to determine themaximum transform size.